The present invention generally relates to technique for recording, reproducing and erasing an information signal with respect to an opto-magnetic recording medium.
Nowadays, there has been developed an apparatus in which information can be recorded on and read out of a recording medium by means of a laser beam, said recording medium being formed by a magnetic film of two-phase amorphous alloy such as GdCo, GdFe, TbFe and DyFe and three-phase amorphous alloy such as GdTbFe. In the case of recording, the magnetic film has been previously magnetized perpendicularly, for instance downward and the laser beam modulated with an information signal to be recorded is projected onto a given portion of the film so as to heat the related portion near a Curie point while the related portion is subjected to an external biassing magnetic field directing upward. Then the direction of magnetization in the relevant portion is inverted. Since the information can be recorded perpendicularly in the magnetic film a very high recording density can be attained. In case of reproducing the information thus recorded in the recording medium, the laser beam is projected via a polarizer onto the medium and the reflected or transmitted laser beam is received by a photodetector via an analyzer. The polarization surface of the linearly polarized laser beam is rotated to some extent due to a magnetic Kerr effect or a magnetic Faraday effect in a direction depending upon the direction of magnetization at the read out portion of the recording medium. By detecting the rotational direction of the polarization plane of the detected laser beam, the information can be read out. In such an opto-magnetic recording medium, not only the recording and reproducing, but also erasing can be effected and thus, the information can be rewritten on ordinary magnetic recording medium such as magnetic tape, magnetic disc and floppy disc.
FIG. 1 is a schematic view showing a known recording and reproducing apparatus using the above mentioned opto-magnetic recording medium. In the case of recording an information signal, a laser beam emitted from a laser light source 1 such as an Ar laser device is modulated in a modulator 2 with the information signal to be recorded. The modulated laser beam is projected by means of half mirrors 3 and 4, a polarizer 5, a lens 6, a mirror 7 and a fly eye lens 8 onto an opto-magnetic disc 10 as a small light spot. The disc 10 comprising a disc shaped MnBi film is rotatably supported by an air bearing 9 and is rotated at a high speed. A part of the modulated laser beam divided by the half mirror 3 is received by a photodetector 11 and an output power of the laser beam from the laser device 1 is controlled in accordance with an output from the photodetector 11. In this manner the information signal can be recorded along concentrical tracks or a spiral track.
In order to detect a position of the recorded information on the disc 10, a laser beam emitted from a laser light source 12 such as He-Ne laser device is projected by means of a mirror 13, a half mirror 14 and a lens 15 onto the same track on the disc 10 as that on which the information signal has been just recorded, and the laser beam reflected by the disc is received by a photodetector 16 via the lens 15 and the half mirror 14.
As described above, the recording of the information signal on the opto-magnetic disc 10 is effected by irradiating a relevant portion of the disc with the modulated laser beam to heat the related portion near the Curie point and the direction of magnetization of the relevant portion is inversed by externally applied biasing magnetic field. FIGS. 2A to 2C show schematically how to record the information signal on the disc. Prior to the recording, a magnetic film 21 of the disc 10 is uniformly magnetized in a direction shown by arrows in FIG. 2A, i.e. upward. When the laser beam 22 is projected onto a given restricted portion 23 of the film 21, the irradiated portion 23 is locally heated. When a temperature T.sub.M of the irradiated portion 23 is increased higher than the Curie point T.sub.C, the portion 23 becomes a paramagnetic condition and the magnetization in the relevant portion is forcedly oriented in a direction of a demagnetizing field Hin produced by adjacent portions of the film 21. Under such a condition, when an external magnetic field Hex having the same direction as the demagnetizing field Hin is applied to the film 21, the relevant portion 23 is magnetized in the direction opposite to that of the uniform magnetization after the temperature T.sub.M of the portion 23 is decreased sufficiently lower than the Curie point T.sub.C as illustrated in FIG. 2C. Since the heated portion 23 is under the paramagnetic condition, the external magnetic field Hex may be sufficiently lower than a coercive force Hc of the magnetic film 21. Further, if the film 21 has a large saturation magnetization, the external magnetic field Hex may be omitted. Such a small external magnetic field Hex does not affect the magnetization of the magnetic film 21 except for the relevant portion 23. It is one of the important merits of the opto-magnetic recording that the magnetization can be inverted by the weak external magnetic field.
In order to reproduce the information recorded in the opto-magnetic disc 10 in the apparatus shown in FIG. 1, the laser beam emitted from the laser light source 1 is focussed on the disc 10 and the reflected light is received via a half mirror 25 and a polarizing prism 26 by photodetectors 27A and 27B. The information signal can be regenerated by supplying output signals from the photodetectors 27A and 27B to a differential amplifier 28.
A light beam reflected by the half mirror 25 is received by a photodetector 29 which produces a signal representing the position of the laser beam spot on the opto-magnetic disc 10.
In the opto-magnetic apparatus shown in FIG. 1, in order to erase the information once recorded on the opto-magnetic disc 10, a laser beam 22' is projected on a portion 23' of the magnetic film 21 in which portion the information signal to be erased has been recorded and at the same time an external magnetic field Hex having a magnitude greater than the demagnetizing field Hin and a direction opposite to the demagnetizing field Hin is applied as illustrated in FIG. 3A. When the portion 23' is heated higher than the Curie point T.sub.C, the magnetization in the portion 23' is forcedly inverted in the same direction as that in adjacent portions. After the temperature of the portion 23' is decreased, the magnetization in the relevant portion 23' is reversed in the same as that in the adjacent portions and in this manner the information can be erased as shown in FIG. 3B. The magnetic film 21 is made of MnBi, CoP, etc. having a very large saturation magnetization. For instance, MnBi and CoP have the saturation magnetization represented by 4.pi.Ms up to about 7,000 and 17,000 Gausses. Therefore, the amount of the external magnetic field Hex for erasing should be correspondingly large and this causes practical problem.
As described above, the information signal is recorded along the track on the opto-magnetic disc and thus, in order to position accurately the light beam spot on the track, focussing and tracking servo mechanisms must be provided. However, when the erasing is effected by orientating the magnetization into the same direction as that in adjacent regions, the track has been completely erased and thus a given tracking signal could not be obtained during a next recording. It should be noted that in case of re-recording, the disc must be erased partially and new information is recorded in the erased portion. Therefore, if the track is completely erased and any tracking error signal could not be derived from the erased portion, an accurate re-recording could not be effected.